1. Field of the Invention
This invention relates to a direct current circuit breaker which uses a vacuum current interrupter.
2. Description of the Prior Arts
Electric cars and electric locomotives (hereinafter referred to as electric rolling stock) have inherent in them a possibility that a failure may occur, such as a short circuit, due to a breakdown of an element (a thyristor, a GTO thyristor, or a transistor, for example) used in the main circuit of an inverter or a chopper, such as a ground fault caused by imperfect insulation of some wire in the main circuit, or such as an abnormal current increase resulting from a failure of the control system. If such a failure is left unattended, the equipment will burn. In order to prevent this accident, electric rolling stock have been conventionally equipped with a circuit breaker to cut off an excess current.
However, with the air circuit breaker heretofore used, for constructional reasons, the breaking speed is slow from when an accidental current flowed until the current is cut off, and before the circuit breaker opens itself, it sometimes happens that a circuit breaker in the ground substation of the feeder section where electric rolling stock is located opens the circuit. When the circuit breaker of the ground substation operates, all the electric cars within the feeder section supplied by that substation are unable to receive power, and thus they stop. In other words, the accident in one electric car stops to other electric cars. If such an accident occurs on a line with a congested train schedule, it is easily imaginable that the accident affects not only the electric rolling stock within the feeder section but also the electric rolling stock of other feeder sections.
This is because of the slow breaking speed of the air circuit breaker installed on the electric car, as mentioned above.
Consequently, there has been requirement for a DC vacuum circuit breaker with much higher breaking speed.
As described in JP-A-54-132776, to cut off a direct current is more difficult than to cut off an alternating current because a direct current does not cross a zero point. As a countermeasure, to facilitate cutoff of a direct current, current zero points are created artificially by providing a switching valve (hereafter referred to as a valve or interrupter with a commutating capacitor in parallel therewith and by forming an oscillation circuit (commutation circuit) in combination with the inductance of the circuit. The methods for this purpose are roughly divided into two groups: the reserve charging methods and the no-charging methods.
The reserve charging method is to charge a capacitor and discharge the electric charge stored in the capacitor when opening the interrupter. In this method, oscillation is produced by the capacitor and the inductance of the circuit. Since this oscillation circuit has a pure resistance component, the amplitude of the oscillation decreases exponentially. As the amplitude of oscillation at its early stage passes through a current zero point, the arc current in the interrupter is eliminated, thereby completing the cutoff.
In the no-charging method, on the other hand, a interrupter with negative arc characteristics is used, a capacitor is connected in parallel with this interrupter, and a divergent oscillating current is obtained when opening the interrupter. When the amplitude of oscillation in the diverging direction passes through a zero point, the current is cut off. This method, however, requires a certain length of time before the oscillation grows and passes through the current zero point.
Therefore, it is possible that before this occurs, the circuit breaker of the ground substation operates.
For this reason, a circuit breaker of the reserve charging method is more convenient when it is installed on the electric rolling stock.
A DC circuit breaker of the reserve charging method disclosed in JP-A-54-132776 is described below.
A capacitor is connected in parallel with the interrupter, and a resonance circuit is formed by this capacitor and stray inductance. By this arrangement, however, the current inclination, which is the time differential (di/dt) when the current flowing through the interrupter crosses the current zero point, is so great that it is difficult to cut off the current. As a solution to this, in order to reduce the current inclination, in addition to the stray inductance, an inductance of more than several millihenries (mH) is connected in series with the capacitor.
Let us consider the effect of varying the magnitude of the capacitor and the inductance of the above-mentioned prior art. If an inductance of several millihenries is used, the capacitor will be several thousand to tens of thousand microfarad (.mu.F). Therefore, the capacitor will become very large in size.
The electric rolling stock have their equipment mounted under the floor and above the roof. The space for mounting the equipment is very limited, and if some apparatus is too large, it cannot be mounted on the electric rolling stock.